Recent studies indicate that several classes of experimentally induced memory failure in animals are reversible. Relatively little is known, however, about the stimulus characteristics critical to producing this recovery of memory. Such information is important in order to understand the determinants of retention. The major purpose of the proposed research is to examine systematically the various conditions which control the reactivation of memory and to investigate the types of prior experience which can modulate the effectiveness of memory disrupting events. The manipulations will be assessed in the context of two different paradigms: Retrograde amnesia, in which traumatic events impair retention of newly acquired responses, and infantile amnesia, in which retention loss is associated with ontogenetic changes. By delineating the way in which reactivation or reinstatement of a target response is achieved in these paradigms, the research provides basic information on the nature of storage and retrieval processes in memory.